It is often desirable to employ force excitation methods for ground vibration testing of aircraft and other test structures. The purpose of such testing is to determine the frequency response functions (FRF) of the structure for various excitation points and frequencies. Force-excitation methods include, for example, multi-reference random excitation (e.g., burst random excitation) and sine sweep methods.
Multi-reference random methods are often preferred because the desired frequency response functions (FRF) for all responses and all references can be measured simultaneously, which greatly reduces data collection times. However, random methods are also be undesirable in that the signal-to-noise ratio during testing can be relatively low, leading to unsatisfactory FRF results.
Sinusoidal sweep methods, on the other hand, provide higher RMS input loads and often lead to much cleaner FRF results. Sinusoidal sweep methods, which involve “sweeping” through a range of frequencies, can also used to provide symmetric and antisymmetric excitations to emphasize symmetric and antisymmetric modes, respectively, particularly in aircraft and other structures in which clear symmetry (e.g., reflective symmetry) can be identified. Such sweep tests are undesirable, however, in that they are single-reference and must therefore be run in series, prolonging the overall testing time. For example, if excitation sources (e.g., “shakers”) are mounted on each wingtip as well as each horizontal stabilizer, then a total of four sine sweeps are required: wing symmetric, wing antisymmetric, horizontal stabilizer symmetric, and horizontal stabilizer antisymmetric.
To address the latter issue, a number of multiple-reference sinusoidal techniques have been developed. These techniques, however, tend to be based on multi-reference stepped-sine testing in which the number of shaker loading combinations is equal to at least the number of shakers used. Since these loading combinations are applied consecutively and not simultaneously (or concurrently), testing time is not improved.
It is therefore desirable to provide improved ground vibration testing techniques that can quickly and efficiently collect all FRF data simultaneously for a test structure. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.